This invention relates generally to a method for the remediation of contaminated particulate matter such as soil. In particular, this invention relates to a method for removing metals, metallic compounds, and/or radioactive contaminants from soil by first segregating the contaminated fines from the clean coarse fraction of the soil, separating any vegetation or other debris from the fine fraction of the soil, and then further treating the contaminated fine portion of the soil to remove metals, metallic compounds, and/or radioactive contaminants using density and/or paramagnetic separation techniques.
Contaminated soil is becoming an increasing environmental problem. Soil may be contaminated with a variety of materials. For example, typical contaminants include, but are not limited to, metals such as copper, lead, and mercury; radioactive contaminants, such as uranium, thorium, or radium; and organics. Metals, metallic compounds, and/or radioactive species are often found in silt, humic, or clay fractions of the soil. In addition, radioactive species may be found in areas where there had been nuclear testing, non-nuclear armaments testing, and mining.
With the environmental problems on the rise, the disposal of contaminated soil is becoming an increasingly important issue. Methods for decontaminating soil are generally disclosed in U.S. Pat. No. 5,128,068, issued to Lahoda et al., on Jul. 7, 1992 (hereinafter "Lahoda patent"), the disclosures of which are incorporated herein by reference in their entirety to more fully describe such methods. Soil washing techniques, such as those described in the Lahoda patent are effective at removing the contamination from soil using physical and chemical means.
A method for separating radioactive and hazardous metal contaminants from soil is disclosed in U.S. Pat. No. 4,783,253 issued to Ayres et al. on Nov. 8, 1988. The method consists of creating a suspension of the soil particles in a column of water. Water is alternately forced up the column to remove the lighter uncontaminated particles, while the heavier particles settle on the bottom. The contaminated heavy soil particles are collected and handled for waste storage.
The method described in U.S. Pat. No. 4,783,253 is limited to those cases when the contamination is present as discrete particles at least 100 microns in size. The method also will not deal with contamination that is associated with or attached to the soil fraction, nor will it work on contamination that is present as particles less than 100 microns in size.
Other methods for treating soil containing radioactive contaminants are disclosed as set forth in U.S. Patent Applications assigned to a common assignee hereof, entitled "Method For Remediating Soil Containing Radioactive Contaminants," U.S. patent Ser. No. 997,076. This patent application discloses a method for remediating uranium and radium contaminated soil by selectively removing the radioactive contamination without removing desirable organic materials that enrich soil and promote plant growth. This remediation technique is based primarily on treating the soil using chemical techniques, such as acid-base and oxidation reactions. Using this method, a leachate solution results that may be amenable to further processing or recycling.
Methods and apparatus employing chemical and physical separation techniques for soil remediation have been described in the Lahoda patent and in U.S. Pat. Nos. 5,268,128 and 5,316,223, both entitled "Method and Apparatus for Cleaning Contaminated Particulate Material," assigned to a common assignee hereof. Generally, these references relate to methods and an apparatus for cleaning particulate matter such as soil through a combination of leaching, washing, attrition scrubbing, countercurrent flow size separation, and physical and/or chemical separation techniques.
Processes such as gravity and magnetic separation techniques have been used successfully in the mineral industry to concentrate metals ores. See, e.g., B. A. Wills, Mineral Processing Technology, Pergamon Press, 1988. For example, gravity techniques have been employed commercially to remove high density metals, such as gold and uranium from ores. Magnetic separation techniques have been employed commercially to separate iron minerals from ore and impurities from clays. Metal cations tend to irreversibly bind to the clay fractions of the soil making disposal problematic. Thus, it is desirable to use such techniques to process soil containing metal, metallic compounds, or radioactive species. However, these processes are not directly transferable to the treatment of soil.
Soil has certain characteristics that do not make it amenable to remediation in these systems. In particular, the soil's nonuniformity makes it difficult to apply these technologies directly. For example, the presence of vegetation and oversize materials in soil, and the adherence of the contaminants onto the soil make soil processing difficult when using gravity and magnetic separation systems. The key to successfully implementing the gravity and magnetic processes disclosed above is the ability of these techniques to handle the soil.
The treatment of fines is particularly problematic because the soil fines tend to contain large amounts of vegetation, particularly root hairs. Root hairs present several problems. First, there is presently no technology available that can decontaminate root hairs, so it is critical that the root hairs be separated from the soil fines. After the root hairs are separated they are typically buried in their contaminated form or incinerated. Second, the root hairs are very light and are not readily treatable using size separation technology, such as mineral jigs. The root hairs tend to float on top of the liquid present in the jig and as a result there is no segregation of contaminants from the root hairs. Third, the root hairs similarly cannot be treated using other physical separation techniques, such as a density separator, because again, they are too light. When root hairs are placed in a density separating device they typically float to the top and are not affected by the force fields. Fourth, the root hairs cannot be decontaminated using chemical techniques such as precipitation or flocculation because the root hairs tend to form a jelly-like mass when treated with the chemical solvents necessary for performing these steps. Once the root hairs become coagulated, they are difficult to handle and treat, which makes further processing essentially impossible. Finally, certain metals and radioactive species tend to get bound up in the membranes of root hairs. Eventually this affects plant growth and ultimately the food chain because these contaminants tend to move through the food chain. See, e.g., Richard Headstrom, Adventures With A Microscope, p. 160 (Dover Publications, Inc., New York 1941); K. W. Brown & Associates, Inc., Irrigation with In Situ Uranium Mining Reclamation Water: Evaluation and Design (August 1982). So it is essential that root hairs be removed from soil streams so the remaining soil may be decontaminated.
In addition, it is not beneficial from an economic standpoint to make all the soil fed to a cleanup process amenable (e.g., by grinding and crushing) to these separation techniques because this would produce a soil product with a very large fines content, making handling difficult, and making the final liquid content too high for land reuse. Large amounts of fines producing pretreatment would also be very costly. Finally, the above mentioned processes for treating fines tend to have much larger costs than do soil washing processes that treat the larger particles.
So there still remains a need for a method that can effectively remove contamination and vegetation from all size fractions of soil, and thus produce a lower volume waste stream and a greater fraction of clean soil.
At the same time, such a method should be economically efficient and be able to interface with soil cleaning methods which can readily remove the larger, non-contaminated soil fraction.